chan_sip: Fix small behavioral change accidentally introduced in r369750

chan_sip: Fix small behavioral change accidentally introduced in r369750

When removing the warning for AST_CONTROL_FLASH from sip_indicate, I also
inadvertently changed the return value, which would likely make the indication
not be sent in audio. This fixes that while still removing the warning message.
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Merged revisions 369792 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369793 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369794 65c4cc65-6c06-0410-ace0-fbb531ad65f3